Mother’s Day is for many families a touching celebration of the contribution of mothers to their families and society as a whole. But the assumptions underpinning the day, that gender is relevant in parenting and that mothering is different from fathering, are becoming increasingly controversial.

Indeed, some have questioned whether or not Mother’s Day is still relevant in modern society.

In the UK this year, The Guardian reported complaints by male same-sex parents about the prominence of Mother’s Day in schools.

And there has been a push this year for Mother’s Day to include celebration of two-mother families, and for Father’s Day to include celebration of two-father families. This is obviously logically inconsistent: either motherhood and fatherhood are different, complementary things to be celebrated (in which case the concepts of Mother’s Day and Father’s Day make sense), or they are not. If we accept that any distinction in parenting based on gender is completely arbitrary, then Mother’s Day and Father’s Day would have to be replaced with a generic “Parent’s Day.”

Obviously, there have always been motherless and fatherless families. Divorce, family breakdown and other circumstances result in this. But the creation of families with the express intention of children not having a mother or father is a different concept entirely.

Legalising same-sex marriage would abolish in law and in culture the distinction between mothering and fathering, and the ideal of a mother and father for every child. Many countries, such as Spain and Canada, after redefining marriage have taken the next step of removing the words “mother” and “father” from birth certificates and replacing them with “Parent 1” and “Parent 2.”

Rightly or wrongly, the traditional definition of marriage upholds a special social status for both motherhood and fatherhood. This is what is at stake in the long-term in the debate over redefining marriage.

Is motherhood dispensable? Is fatherhood dispensable? Some people say yes. But one thing is certain: we can’t celebrate motherhood and then at the same time support policies which deliberately undermine a child’s right to a mother.

Blaise Joseph is a third-year commerce student at the University of New South Wales with a strong interest in social policy. Blaise is originally from Canberra, the centre of politics and the public...